Control system documentation

One of the basic activity in any project is to ensure it is well documented. 
Automation and control projects are no exception, though they have different and unique requirements to properly capture the requirements, outcomes, and deliverable of the work being performed. 

The International Society of Automation (ISA) has developed standards that are broadly accepted and used across the industry as the preferred method for documenting control systems.

The purpose of this post is to highlight and present briefly the major instrumentation and control documentation required for successful project execution.

The information presented in this post is according to relevant standards from ISA (International society of automation).

Why do we document?

Automation and control solutions become more complex and involve the inputs of and coordination with different disciplines such as process, Mechanical, Electrical,...

Documentation is used to define the control system design, construction  and implementation  by defining the work to be done throughout the project life cycle.

Documentation Types

Below is list of main documents that should be developed during the design and implementation of a control system project by the main automation contractor:

• Piping and Instrumentation Diagram (P&ID)
• Instruments list
• Specifications forms
• Logic Diagrams
• Instruments Layout
• Installation details
• Loop Diagrams
• Control Philosophy

Other documents, such as control system architecture, Input / Output list and Control panel wiring diagram, are not described under this post as these documents are usually prepared by Panel builders and System integrators and will be subject to a separate post.

The time interval and percentage of effort of each task do vary by industry and by designer. 

Nevertheless, below figures can be considered as guidance for the typical % allocation of a control system engineer design hours:

• Piping and Instrumentation Diagram (P&ID): 20%
• Instruments list: 5%
• Specifications forms: 20%
• Logic Diagrams: 10%
• Instruments Layout: 5%
• Installation details: 5%
• Loop Diagrams: 20%
• Control Philosophy: 15%

Next sections will comprise definition and brief descriptions of the above listed documents. Each document type will be tackled in details in separate posts

Piping and instrumentation diagram (P&ID)

The P&ID is the principle document used to define the equipments, piping, instrumentation and control loops used in a process.

The international society of automation (ISA) defines P&ID as follows: "Show the interconnection of process equipment and instrumentation used to control a the process".

While the PFD (Process flow diagram) shows what the process will do, the P&ID defines how it happens.

Below figure shows a sample P&ID including control system definition using symbols according to ISA-5.1 & ISA-5.3

Source: ISA

P&ID is developed is sequence by various design disciplines. Information placed by one discipline is then used by other disciplines as the basis of their designs

A unique loop diagram is used to group a set of functionally connected process control element. Grouped element generally includes the measurement of a process variable, an indication or control element.

Letters combined with the loop number comprise a tag number. Tag number is a unique identifier for each component of a P&ID

Below figure shows an example of pressure loop

Source: ISA

Instruments List

Instruments list is also called Instrument index.

ISA-5.1 describes instruments List or index as follows:
"An instrument index should contain references to all instrumentation data required by owner and contain as a minimum for each loop:

• Loop identification number
• Service description
• Instrument identification / Tag numbers
• Piping and instrumentation diagram drawing numbers
• Instrument data sheet numbers
• location plan drawing numbers
• Installation detail drawings number"

Below is a typical Instrument list template sample according to ISA-5.1 definition:

Usually instruments list is customized by project design team to support the project needs and requirements

Specifications forms

Specification forms are also know as "Datasheets". The Specification forms include useful information, on each tagged device in a P&ID, for designer and suppliers during the design and procurement phases of the project. It enables vendors and suppliers to quote for the correct device.

These forms are also useful for operation and maintenance personnel after the handover of the project.

The form lists all the components information such as materials, ratings, area classifications, range, signal, power, service, etc...

Each tagged device shall have its own specification form or datasheet

The International society of Automation (ISA) has developed set of specification forms as per ISA Publication reference ISA-20-1981: "Specification forms for process measurement of control instruments, primary elements and control valves"

Below is a sample of specifications form for the Level transmitter:

Source: ISA

Logic Diagram

Logic diagram is a presentation for On/Off control.

Symbols of logic diagram are defined in ISA-5.2-1976 "Binary Logic diagrams for process automation"

Below figures show ISA symbols AND, OR, NOT, and MEMORY (Flip - Flop) with their explanation:

Source: ISA

Other methods may be used to document On/Off control such as ladder diagram and electrical elementary diagrams

Below figure is a illustration of a motor start circuit as an elementary diagram and in ISA Logic format

Source: ISA

Instruments layout drawing

This drawing shows the location of the instruments on the plan drawings of a plant. These drawings are useful for people building the facility and can be used by operation and maintenance team as a road map for the system.

Installation details

Installation details are also known as hookup details, define the requirements for proper installation of instruments.

The installation details show process connection, pneumatic tubing, conduit connections and support method.

There is no ISA standard that defines installation details. The installation details shall follow the manufacturer recommendations and the project specifications requirements. At the end the installation details aim to ensure successful installation of the instrument so that it operates properly.

Below figure shows one approach for installation details

Source: ISA

Installation details may be developed during design phase but most of the time, are developed by the installer during construction

Loop diagrams

ISA defines a loop diagram as "Schematic representation of a complete hydraulic, electric, magnetic or pneumatic circuit". The circuit is called as loop.

Below figure shows a typical loop diagram according to ISA-5.4-1991

The loop diagrams are helpful for maintenance and troubleshooting as they show the complete connection loop from process to the control device all in one sheet.

Operating instructions

Operating instructions also known as "Control narrative" or "Control philosophy", are needed for operation of complex plant.

They range from a few pages describing how to operate a part of the of a plant to a complete set of books covering the operation of all parts of a facility.

There is no ISA standard to aid in developing operating instructions. The development of such document involves different disciplines such as process, mechanical, electrical, instrumentation and automation during the plant design

The operating instructions describes how the plant shall behave in normal operation, in abnormal operation, during startup, shutdown and emergency operation.